A conventional rear hub driving apparatus for a bicycle, as shown in FIGS. 15 and 16, includes a hub shell (40) and a ratchet plate (50), wherein the hub shell has a receiving space (41) having a shaft (42) passing through, and the shaft (42) has locking unit (421) on both ends. The receiving space (41) has a fixed base (43) at an inner perimeter thereof, and the fixed base (43) has a plurality of protruding ribs (431) located around an inner perimeter to engage an action ring (44) that has a plurality of square wedging slots (441) at an inner perimeter. The ratchet plate (50) has a tooth plate (51) and a ratchet base (52) on each side for a chain, wherein the ratchet base (52) is located at the receiving space (41), and the ratchet base (52) has a plurality of engaging slots (53) at an outer perimeter that has a spring (531) and a ratchet (532). The spring (531) is against an inner portion of the ratchet (532), so that the ratchet (532) is against the wedging slot (441) of the action ring (44). When the bicycle moves forward to drive the ratchet plate (50), the ratchet (532) is against the wedging slot (441) of the action ring (44) to drive the hub shell (40) to rotate. When the ratchet plate (50) stops rotating, the ratchet (532) detaches from the wedging slot (441) and presses the spring (531), so that the status between the hub shell (40) and the ratchet plate (50) can be switched when the user pedals. According to different riding habits, when the user may want to switch the chain from right to left side, the user can swap the spring (531) and ratchet (532) in the engaging slot (53) of the ratchet base (52) to obtain a bicycle with the chain on the left side.
Some disadvantages in the abovementioned conventional structure can be summarized as following:
(a) The ratchet (532) is pushed by the spring (531) to be against the wedging slot (441) of the action ring (44) to drive the hub shell (40) to rotate, and the wedging slot (441) of the action ring (44) is square, so the contact area of the ratchet (532) is too small, which may lead to insufficient structural strength.
(b) The wedging lot (441) of the action ring (44) is square to increase the contacting height of the ratchet (532), namely enlarging the volume of the action ring (44), which may increase the swing magnitude of the ratchet (532) on the action ring (44) and reduce the flexibility thereof.
(c) The action ring (44) is located at the fixed base (43) of the hub shell (40), and axially positioned through the inner wall of the hub shell (40) and the ratchet plate (50). However, there is still a gap between the action ring (44) and the ratchet plate (50), so that the action ring (44) still has some room for axial movement that may cause wear between the ratchet plate (50) and action ring (44) to reduce durability of the structure of the hub shell.
(d) The single-directional driving style is only suitable for regular bicycles, but not suitable for performing stunts that may need to drive the tires going back and forth simultaneously. Thus, the cost for purchasing the bicycle is higher. Moreover, since the hub shell has low applicability, the manufacturer has to produce different kinds of hub shells and corresponding components, so the manufacturing costs can be higher and more inventory can be expected. Therefore, there is a need for a new and improved hub shell structure for the bicycle to overcome abovementioned disadvantages.